In recent years, it has been reported several times about monkeys developing horrifically aggressive behavior. The reason for the monkey attacks is apparently normal behavior of animals, while their submissive behavior the rest of the time is a disturbance created by human influence.
Investigations of attacks on macaques and langur monkeys in Asia and India point to over-adaptation to the presence of humans as the main driver of attacks. Backing off from jumping, hitting the ground, or baring teeth is the smart thing to do, even if the movements were only made as an empty threat.